1. Field of the Invention
The present invention relates generally to a grinding apparatus for grinding works such as semiconductor wafers or the like. More particularly, the present invention relates to improvement of a grinding apparatus of the foregoing type which assures that a grinding operation can uniformly be achieved with the aid of a work position aligning mechanism and a displacing mechanism for reciprocally slidably displacing top rings having works absorptively secured thereto. Further, the present invention relates to a polishing apparatus for polishing wafers such as semiconductor wafers or the like.
2. Background Art
A conventional grinding apparatus for grinding a wafer (work) such as a semiconductor wafer or the like is usually constructed such that a wafer to be ground is absorptively secured to a top ring and the wafer is then thrust against a rotary table having an abrasive cloth adhesively placed thereon so as to allow the wafer to be ground while a lubricant slurry is fed between the wafer and the abrasive cloth on the rotary table.
To achieve a grinding operation with the foregoing type of grinding apparatus at a high level of accuracy, it is required that the wafer is uniformly thrust against the abrasive cloth on the rotary table over the whole surface of the latter. To meet the requirement, it is necessary that the center of the top ring is exactly located in positional alignment with the gravitational center of the wafer.
In many cases, however, the semiconductor wafer is not prepared with a completely circular contour, and moreover, an orientation flat part is formed at a part of the outer periphery of the wafer. Thus, a geometrical center of the wafer defined on the assumption that the wafer is prepared with a completely circular contour (hereinafter referred to simply as a center of the wafer) is not positionally coincident with the gravitational center of the wafer. In addition, when the upper surface of the wafer does not extend in parallel with the lower surface of the same, i.e., the wafer does not have a constant thickness over the whole surface thereof, there arises a malfunction that the center of the wafer is positionally offset from the gravitational center of the same when the wafer is absorptively secured to a top ring of the grinding apparatus. This leads to the result that it can not be expected that a grinding operation is reliably achieved at a high accuracy because a certain intensity of thrusting power is not uniformly applied to the wafer.
In the circumstances as mentioned above, a method of properly locating a wafer via calculations conducted in consideration of the relationship between the center of a wafer to be ground and the gravitational center of the same has been already proposed as disclosed in an official gazette of Japanese Patent Laid-Open Publication NO. 2-159722, of which patent application was filed by an applicant common to the present invention.
However, since a grinding apparatus for which the foregoing method is employed is constructed such that a top ring is arranged separately from a work holding table, there arises a problem that mechanical components constituting the grinding apparatus are fabricated and assembled together with many difficulties in order to assure that the central position of the top ring is positionally coincident with the gravitational center position of a work to be ground.
In the case that a polishing operation is performed with the conventional grinding apparatus described above, since the position of the top ring and the center of rotation of a rotary table are always kept constant, the locus along which the wafer comes in contact with an abrasive cloth is transferred to the surface of the wafer, causing ring-shaped ruggedness to appear on the ground surface of the wafer, resulting in a machining accuracy of the polishing operation being degraded. In addition, it has been found that a part of the abrasive cloth on the rotary table corresponding to the ground part of the wafer is locally worn for a short time, resulting in an incorrect polishing operation being acceleratedly performed. Thus, there arises an overly-frequent necessity that the prematurely worn abrasive cloth is replaced with a new one.